Field of the Invention
The invention relates to a semi-finished product for the production of a printed circuit board having a plurality of alternately arranged insulating layers and conductive layers and at least one hard gold-plated edge connector, a method for producing a printed circuit board as well as a printed circuit board.
Description of the Related Art
Printed circuit boards, also referred to as printed wire boards are panels carrying and electrically connecting electronic components such as transistors and the like and, hence, form vital parts of electronic products. Printed circuit boards have a more or less complex structure depending on the specific application. In general a printed circuit board has a plurality of alternately arranged conductive layers and insulating layers bonded together by panels of glass fibres impregnated with organic resin, said panels forming the insulating layers. Such panels for use in the production of printed circuit boards are widely known in the industry as “prepregs” (preimpregnated fibres), which are delivered and processed in an uncured, hence viscous state of the organic resin. The actual insulating layers of the printed circuit board result when the organic resin has cured. The insulating layers carry conductive layers, usually formed of copper foil, the conductive layers being appropriately structured to form wirings to electrically connect the electronic components. Modern printed circuit boards allow for a high degree of integration of electronic components and their appropriate wiring.
In many applications printed circuit boards are produced as exchangeable boards that can be inserted into appropriate jacks of electronic machines, such as, for example, computers. To this end, printed circuit boards have edge connectors, also known in the art as tabs or fingers. Edge connectors of printed circuit boards have to provide low contact resistance as well as high wear resistance which is why they are often electroplated with precious metals. In this context, gold electroplating is commonly used. Since gold is a rather soft metal that would not provide the necessary wear resistance it is alloyed with nickel, cobalt or indium for the purpose just described. This alloy called hard gold is then electroplated on contacts formed of copper which have been obtained previously by known structuring procedures of conductive layers of a printed circuit board. A typical edge connector would feature a 1 μm-3 μm gold-plating with an undercoat of 4 μm nickel on a copper contact. The edge connector together with the insulating layers and conductive layers supporting the edge connector normally has a thickness corresponding to standard thicknesses of industrially used standard jacks so that it can easily be inserted into the corresponding jacks.
While an above-described gold-plated edge connector provides superior contacting and wear properties for contacting a printed circuit board to a jack of an electronic machine it is often necessary to provide contacting substrates having completely different properties for contacting electronic components on the printing circuit board, such as resistors, capacitors, inductors, diodes, transistors, thyristors, integrated circuits and the like. Often these components are contacted to the printed circuit board by wire bonding, a process in which the pins of an electronic component are connected to the pads of the printed circuit boards by means of bond wires. A preferred surface treatment to enable wire bonding procedures is known as “ENEPIG”. ENEPIG stands for “electroless nickel, electroless palladium, immersion gold” and means applying a layer of nickel and a layer of palladium on a pad contact of a printed circuit board to be contacted by wire bonding by an electroless, hence reductive method. Finally, the pad of the printed circuit board covered with nickel and palladium is covered with a layer of gold applied by immersion plating.
The production of printed circuit boards having hard gold-plated edge connectors and at the same time connector pads for electronic components coated with the ENEPIG technology imposes technological difficulties and hence brings about relatively high costs in the production. This is due to the fact that the areas plated with hard gold have to be protected or masked from the chemical substances used in the ENEPIG surface treatment to avoid deposition of the ENEPIG-layer. In the state of the art, the previously prepared hard gold-plated edge connectors were covered with dry film or photoresist lacquer before the printed circuit board was subjected to the ENEPIG treatment. In doing so it is, however, problematic that the dry film or the photoresist lacquer used to cover and protect the hard gold-plated edge connectors from the chemicals used in the ENEPIG treatment is dissolved to a considerable amount into the baths of the ENEPIG treatment line, so that the chemicals were quickly contaminated with the organics of the film or the lacquer and had to be frequently discarded, thereby dramatically increasing the production costs also due to considerable expenditure for environmentally safe disposal of the spent chemicals.
It is thus an object of the present invention to improve semi-finished products as initially described in order to overcome the above-described dissolution problems.